A porous honeycomb structure is widely used as a filter for capturing and removing the particulate substance contained in a particle-containing fluid such as diesel engine exhaust gas, or as a catalyst carrier for loading thereon a catalyst component which purifies the harmful substance present in an exhaust gas. Also, it is known to use refractory particles such as silicon carbide (SiC) particles, as a material constituting such a honeycomb structure.
As a specific technique related thereto, there is disclosed, in, for example, JP-A-6-182228, a porous silicon carbide-based catalyst carrier of honeycomb structure obtained by using, as a starting material, a silicon carbide powder having a given specific surface area and containing impurities, molding the powder into a desired shape, drying the molded material and firing it in a temperature range of 1,600 to 2,200° C.
There is also disclosed, in JP-A-10-310474, a SiC—Si composite ceramic material containing a given amount of Ca or a Ca compound and showing durability in a high-temperature alkali gas atmosphere.
In the sintering (necking) mechanism owing to a recrystallization reaction of silicon carbide powder per se, shown in the above-mentioned JP-A-6-182228, a silicon carbide component vaporizes from the surfaces of silicon carbide particles and condenses at the contact areas (necks) between the particles; as a result, the necks grow and a bonding structure is obtained. However, the vaporization of silicon carbide requires a very high firing temperature, which incurs a high cost, requires high-temperature firing of a material high in thermal expansion coefficient, and has invited a problem of reduced yield in firing process.
Also in production of the SiC—Si composite ceramic material shown in JP-A-10-310474, firing at a high temperature of 1,800 to 2,000° is necessary. Further, this SiC—Si composite ceramic material is a dense material used mainly as a firing container such as crucible or the like, making it impossible to use the ceramic material as a material for porous filter.
In order to solve the above-mentioned problems, the present inventors disclosed, in Japanese Patent Application No. 2001-32699, a porous honeycomb structure containing, as an aggregate, refractory particles, particularly silicon carbide and metallic silicon, and a process for production thereof. In the patent application was disclosed a honeycomb structure which can be produced inexpensively at a relatively low firing temperature, which has a high thermal conductivity, and which is sufficiently porous and has a high specific surface area.
Even with the production process shown in Japanese Patent Application No. 2001-32699, however, a problem may arise. FIG. 4 is a schematic drawing explaining a contact state between silicon carbide particles and metallic silicon, in the fine structure of a conventional silicon carbide-based porous material, and indicates a state in which metallic silicon 2 is in contact with silicon carbide particles 1 and the surfaces of the two materials are each covered with? by an oxide film 3. Incidentally, the oxide film 3 referred to herein is SiO2 or the like. When firing is conducted according to the above process, the wettability between molten metallic silicon 2 and silicon carbide particles 1 may not be good and, in the resulting silicon carbide-based porous material, the contact angle θ between the oxide film 3 on silicon carbide particles 1 and the oxide film 3 on metallic silicon 2 is obtuse. That is, the silicon carbide-based porous material has such a structure as metallic silicon 2 is repelled by silicon carbide particles 1, and the contact area between the two materials may be small.
Thus, when the above-mentioned contact area is small, there may be expected to rise such problems that the strength of the silicon carbide-based porous material per se is reduced, and that the thermal conductivity of the product becomes lower since its path for thermal conduction is narrow.
Meanwhile, the pore diameter of the produced silicon carbide-based porous material depends upon the particle diameter of the silicon carbide particles used as an aggregate. That is, in order to obtain a silicon carbide-based porous material of large pore diameter, silicon carbide particles of larger particle diameter are used generally. Use of silicon carbide particles of large particle diameter may make molding difficult. Further, abrasion of molding die is striking in extrusion into a honeycomb shape or the like, which may invite problems of increased production cost, etc. That is, selection of particle diameter of silicon carbide particles in order to control the porosity of silicon carbide-based porous material and make large the pore diameter thereof has a restriction in technique and production cost.
The present invention has been completed in view of the above-mentioned problems of prior art, and aims at providing a silicon carbide-based porous material which can be produced at a low cost and which is high in porosity and thermal conductivity and is increased in strength, and a process for production thereof.